As is well known in the art, yarns which are used for industrial applications or as materials for shoe uppers are generally made of polyester, nylon, acrylic resin or the like. Fabrics made of such yarns have insufficient durability and abrasion resistance and also problems in terms of adhesive strength and the like, and for this reason, the use of such yarns for high-functionality applications is not suitable.
To solve such problems and increase the strength of yarns, coated yarns obtained by coating the surface of yarns with thermoplastic resins are currently being used. Such coated yarns are generally produced by coating yarns (such as polyester or nylon yarns) with thermoplastic resin such as polyvinyl chloride (PVC), polypropylene (PP) or thermoplastic polyurethane in the dice of a conventional extruder.
However, where conventional thermoplastic resin as described above is used, there are disadvantages in that it is difficult to control the amount of coating with this thermoplastic resin, and particularly, it is difficult to apply this thermoplastic resin in small amounts, and thus the use of this resin can provide only coated yarns having a thickness of 300 denier or more, which also have poor durability or abrasion resistance.
In order to solve such problems, up to date since 2012, the present inventor has conducted studies on a method for production of a coated yarn, a compound for a coated yarn, a thermoplastic polyurethane-coated yarn, etc. (see Patent Documents 1 to 4 below).
The inventions disclosed in such Patent Documents make it possible to produce a coated yarn not only exhibiting excellent abrasion resistance, adhesive strength, waterproof ability and moldability, but also having a relatively thin thickness. However, in the case of coated yarns having a core formed of polyester or nylon, there is a limit to reducing the thickness of the coated yarn, and thus it is impossible to produce a coated yarn having a very thin thickness, preferably a thickness of 50 denier or less.
In addition, in the case of a thermoplastic polyurethane yarn (TPU)-coated yarn disclosed in the prior art patent documents, a thickener is necessarily required for smooth extrusion, because the thermoplastic polyurethane has a lower viscosity than polyester or nylon.
Unlike the TPU-coated yarn, an uncoated yarn such as a mono-filament yarn having a large thickness can be produced using silica having a general size as a thickener. However, in the case of a mono-filament yarn or multi-filament yarn having a small thickness, particularly a thickness of 50 denier or less, general silica powder cannot be applied, because the yarn should be produced to have a small thickness.